This invention relates to a receiver for use in a digital communciation network to receive a digital signal which is supplied through a digital communication path.
When digital communication is made through such a digital communication path, a sequence of digital signals which is conveyed by a transmission carrier and which is transmitted as a modulated signal is often subjected to an intersymbol interference during transmission in the digital communication path, as is known in the art.
On the other hand, a receiver of the type described usually comprises a coherent detection circuit which carries out coherent detection to demodulate the digital signals by the use of a reproduced carrier derived from the modulated signal. On such coherent detection, the reproduced carrier is not always completely identical or coincident with the transmission carrier due to incompleteness of the coherent detection circuit. When noncoincidence takes place between the transmission carrier and the reproduced carrier, a frequency offset inevitably occurs in the receiver device and makes it difficult to accurately detect reception signal points and to demodulate the digital signals with a high reliability because the frequency offset brings about rotation of the reception signal points due to the frequency offset. In addition, such a frequency offset is varied in dependency upon the intersymbol interference. This shows that the intersymbol interference should be eliminated together with the frequency offset in order to realize digital signal transmission with a high quality.
Heretofore, a wide variety of methods have been proposed so as to avoid degradation of characteristics due to the intersymbol interference. For example, there are known a linear equalization method and a maximum likelihood sequence estimation (MLSE) method which is described by J. G. Proakis in "Digital Communications" New York: McGraw-Hill, 1983, Chapter 6. At any rate, the intersymbol interference is equalized in the above-mentioned methods by estimating an impulse response of a transmission path or communication path from a reception signal to produce an estimation result, by calculating, by the use of the estimation result, a mean square value of error signals between input and output signals of a detector, and by minimizing the mean square value. With such methods, it is possible to remove the intersymbol interference of the transmission path and to thereby avoid degradation of reception characteristics only when the impulse response is kept invariable during receiving the digital signals. However, when the frequency offset takes place in the receiver, phases of the reception signal points are rotated in dependency upon the frequency offset, namely, a difference between the transmission carrier and the reproduced carrier even when the intersymbol interference is eliminated due to the above-mentioned manner. Such phase rotation of the reception signal points may be considered as being equivalent to a variation of an impulse response of the transmission path.
Herein, a proposal has been offered about a method of estimating such a frequency offset to eliminate the same in a paper which is contributed by T. Osawa and M. Iwasaki, to IMSC 90 Conf. in Ottawa and which is titled "Performance Evaluation of a Mobile Satellite System Modem using an ALE Method". With this method, the frequency offset is estimated after a reception signal is frequency multiplied to remove a modulated component. According to this method, it is not so difficult to estimate the frequency offset when no intersymbol interference takes place. However, such a frequency offset can not be precisely estimated on occurrence of any intersymbol interference.
Under the circumstances, it is difficult to realize digital signal transmission with a high quality by both the conventional techniques of removing the frequency offset and intersymbol interference when both the intersymbol interference and the frequency offset concurrently take place.